H52 standard reference hydrophone

ABSTRACT

An electroacoustic transducer with features permitting a structure of minimum size to thereby permit reflection and absorption measurement to be made over a broad band of frequencies without disturbing the sound field. A plurality of lithium sulfate crystals are bonded to gold plated silver foil electrodes with an isolating space being provided between each adjacent pair of crystals. The crystal assembly is supported by compliant members within a wire frame which is surrounded by an expanded metal electrostatic shield. The whole assembly is enclosed within a castor oil filled butyl boot. Output terminals are connected through a hermetic-seal-through to a conventional preamplifier. Cable connections to the preamplifier are made through a pressure resistance coupling of unique design.

Kieser et al.

[ 1 H52 STANDARD REFERENCE HYDROPHONE [75] Inventor: RobertJ. Kieser;Allan CQTiins,

both of Orlando, Fla.

[73 Assignee: The United States of America as represented by theSecretary of the Navy [22] Filedf Feb. 24, 1972 '7 WWW M mum \M- [21]Appl. No.: 228,833

[52] US. Cl I. LI.IIITILjLIjl'liji. ..'I IQ:BZWE [51] Int. Cl. H04b13/02 [58] Field of Search 340/8, 9, 10, 12, 340/13 [56] ReferencesCited UNITED STATES PATENTS 3,432,000 3/1969 Ongkiehong et a1 340/8 LF2,613,261 10/1952 Massa 340/10 3,054,982 9/1962 Kieser 340/10 2,490,23612/1949 Shaper 340/10 X 3,027,540 3/1962 Howatt 340/10 PrimaryExaminer-Benjamin A. Borchelt [111 3,760,348 Sept. 18, 1973 AssistantExaminerl-l. J. Tudor Attorney-R. S. Sciascia et al.

[ 5 7] ABSTRACT An electroacoustic transducer with features permitting astructure of minimum size to thereby permit reflection and absorptionmeasurement to be made over a broad band of frequencies withoutdisturbing the sound field. A plurality oTlithiurn s filfate crystalsare bonded to gold plated silver foil electrodes with an isolating spacebeing provided between each adjacent pair of crystals. The crystalassembly is supported by compliant members within a wire frame which issurrounded by an expanded metal electrostatic shield. The whole assemblyis enclosed within a castor oil filled butyl boot. Output terminals areconnected through a hermetic-seal-through to a conventionalpreamplifier. Cable connections to the preamplifier are made through apressure resistance coupling of unique design.

6 Claims, 13 Drawing Figures BACKGROUND OJF TI IE INVENTION Thisinvention is directed to piezoelectric type transducers or hydrophonesand more particularly to ahydrophone which is' s'aar;ravages-Basia:

band, temperature stable and is operable in a frequency range of from Hzto 500 KHz at a pressure up to 7,500 psig.

Heretofore, hydrophones that are available for operation at great depth(high pressure) are large, cumbersome, and of limited frequencyresponse. Large hydrophones are unsuitable for reflection and absorptionmeasurements because they disturb the sound field in which themeasurements are made. Other hydrophones that embody cellularsoundabsorption material are undesirable because their sensitivity issubject to change with temperature or pressure, or both. Such prior artdevices areset forth in US Pat. Nos. 2,413,462; 3,054,982 and'3,118,125; also in an article-Standard Hydrophone for the InfrasonigandAudio-frequency Range at Hydrostatic Pressure to 10,000 psig by T. A.Henrequez and ..EZ: -l.- 1L9l. fil .Saeisty 9f Am'r'iclfi pp. 276-280,Jan. 1970.

Tiie'hydroplion '6f'tiiisih7e'ntion may Be'u ed'as a reference standardthrough a wide range of frequency, temperature, and hydrostaticpressure. It

is small in size, light weight, and has an acoustically transparenthydrophone head which makes it par-- ticularly useful in reflection andabsorption, measurements. In the frequency range 201-12 to 500 KHZ, the

receiving sensitivity changes by less than one decibel at pressures to6,895 kPa (1,000 psi) gage. The hydrophone head is formed by a pluralityof rectangular lithium sulfate crystals bonded. together and connectedelectrically in parallel by suitable electrodes. The crystal assembly issupported within a wirelike frame by suitable rubber mounts. An expandedmetal cylinder having diamond shape apertures formed therein surroundsthe wirelike frame and serves as an electrostatic shield. Theelectrostatic shield is surrounded by a butyl boot of uniform crosssection and the head assembly confined by the boot is filled with castoroil. The output of the crystals is directed into a well knowntransistorized preamplifier and then out through suitable connectionsthrough a Y suitable cable to a desired receiver.

STATEMENT OF THE OBJECTS It is therefore an object of the presentinvention to ductivity.

Other objects and advantages of the invention will become obvious byreference to the following description of the drawings.

BRIEF DESCRIPTION OFTHE o-RAwiN'os H52 STANDARD REFERENCE HYDROPH'ONE'FIG. 1 illustrates a side view of the crystal assembly.

FlG. 2 illustrates a top sembly.

FIG. 3 illustrates one end of the crystal assembly. FIGS. 4 and 5illustrate a side view ofThe crystal assembly.

view of the crystal as- FIGS. 6, 7, and 8 illustrate typical directivitypatterns in the XY plane;

FIGS. 8, 10, 11, and 12 illustrate typical directivity patterns in theX2 plane; and

FIG. 13 illustrates a typical free-field voltage sensitivity of thehydrophone.

DESCRIPTION OF THE PREFERRED EMBODIMENT Now referring to the drawings,FIGS. 1, 2, and 3, i :there is shown by illustration a crystal assemblyfor use in a high'pressure hydrophone.

As sliown, the crystal assembly includes eight lithium sulfate crystals1 assembled in two rows of four each with their adjacent end facesspaced from each other by a small separation 5. The rows of crystals arealigned one over the other and are bonded together and connectedelectrically in parallel by gold-plated silver foil electrodes 2 bondedto their outer faces. The electrodes 2 are joined at one end and areconnected electrically to a low potential terminal 3. The inner adjacentfaces of the crystals are bonded to an electrode 4 which forms the highpotential terminal. As shown in FIGS. 2 and 3, one end of each of theelectrodes is cut at an angle su l g t hat when the ends of electrodes 2are bent -toward each other to be coniiected together at 3 as shown inFIG. 1, they do not touch the center electrode 4.

FIGS. 4 and 5 illustrate a sectional view of the hydrophone head andelectrical connections, with the crystal assembly in place within thehead assembly. As shown, the crystal assembly is supported within a wireframe or cage which comprises two U-shaped wires secured together withthe wires at relative to each other with their ends secured such as bywelding to a cylindrical adapter 9. The wire frame is held together andsupported by reinforcing rings 8 which provide mechanical strength forthe wires. The crystal assembly is mounted coaxially within the wirecage and held in place by suitable thin natural gum rubber mounts 6which are bonded within the wire frame or cage 7. A flattened,expandednickel cylinder 15 having a thickness of about 0.0051 cm whichforms an electrostatic shield for the crystal assembly is secured aboutthe wire cage and is point soldered to electrical ground through adapter9. The expanded cylinder 15 has diamond shaped openings therein due tothe expansion thereof through which castor oil 17 is free to move. Aclosed end butyl boot 16 of uniform cross section surrounds the expandedcylinder and confines the castor oil within the hydrophone head aboutthe crystal assembly for acoustic coupling between the crystal assemblyand the boot as is well known in the art. The open end of the boot isvulcanized and bonded to a cut down end portion of a stainless steelelongated cylinder 18 which is screw threaded onto a hydrophone headzztgapter assembly 11 and held in place by set screw Adapter 9 to whichthe cage is secured is secured to the hydrophone head adapter assembly11 by an overlapping section and is secured thereto by a set screw 10.The adapter 9 is provided with a shoulder that rests against the end ofadapter assembly 11 which prevents further movement of the adapter ontothe assembly 11. An o-ring 19 is positioned between the head adapterassembly and the stainless steel cylinder 18 to avoid leakage of castoroil from 5 the hydrophone head assembly. Since the boot is secured tothe stainless steel ring which is screw threaded onto the hydrophonehead adapter assembly, the boot may be removed from the assembly byremoving the set screw 20 and then unscrewing the stainless steelcylinder.

The hydrophone head adapter assembly 11 is-of cylindrical constructionhaving a closed end near the adapter 9 through which are locatedparallel apertures which receive therein hermetic-seal elec- 15 tricalconductive feed through 12. The opposite end of the head assembly isopen and receives therein one end of a coaxial connector 13 which seatsagainst a shoulder on the inner surface of the assembly. The open end ofthe assembly is provided with threads on the outer surface thereof uponwhich a stainless steel cylindrical housing 21 is threaded. The end ofthe stainless steel housing rests against an outwardly extending n'b onthe adapter assembly and the outer surface of the adapter assembly isprovided with a groove within which an o-ring 22 seats in order toprovide a seal between the end of the stainless steel housing 21 and thehydrophone head assembly 11. The stainless steel housing 21 houses thepreamplifier section including therein a conventionaltransistorizedpreamplifier 14 such as set forth in the above citedarticle.

Suitable electrical conductors are secured to electrodes 2 and 4 and theelectrical feed throughs 12. Also electrical conductors are used betweenthe electrical feed throught 12 and the coaxial connector 13 which inturn connects electrically with the preamplifier 14. The preamplifier 14amplifies the output developed by the crystals which is directed throughan output connector 23 through a hermetic-seal 4 header 36 and a cableterminal 31 to the cable 24 which is connected to any desired suitableequipment which is well known in the a tr The conductor cable 24 issecured to an 'end connector 32 by use of a metal ferrule such as coppera 34 which is silver soldered to the inner surface of one end of the endconnector 32 and swaged 33 to the cable 24. The end connector 32 issecured to the preamplifier section stainless steel housing by user ofscrew 26, an o-ring 25 prevents any leakage between the joint. Thehermetic-seal header is secured to the end of the end connector 32 andis provided with posts 41 which plug into the preamplifier outputconnector 23. Wire conductors 35 are connected to the opposite ends ofposts 41 and are electrically connected to the conductors 29 of thecable by use of a cable terminal 31 which is made of a hard, paperbasephenolic laminate material with good mechanical and electricalproperties. The cable terminal 31 is precision tapered to fit into theend of the end connector 32 and is provided with apertures 42 thereinwhich receive therein eyelets 30 that are soldered to the conductorwires 29. The eyelets are provided with shoulders 43 which preventthe'eyelets and con- ,65 ductors from being forced through theapertures. The space within the end connector through which theconductor wires 35 pass is filled with a hard set- -x... ting epoxy tohold thewires and the posts 41 in place while giving mechanical strengthto the cable terminal.

A cable spacer 28 is provided between the end of the 4 cable and thecable terminal through which the cable wire conductors pass. Aninsulator protective neoprene rubber covering is vulcanized to theferrule 34, the end of the cable near the ferrule and to the end of theend connector 32.

In assembly, the hydrophone head, the hydrophone head adapter assembly,and the preamplifier section may be assembled separate from the cable,cable end connector and cable terminal assembly and sub- .ssseee lxi ieitsrethstblrlussins the on te post 41 into the preamplifier outputconnector and securing the screw 26 in place which holds the sectionstogether.

The transducer may be assembled in three separate sections. Thehydrophone head assembly 44 which includes crystal assembly; thepreamplifier section 45 which includes the preamplifier and itselectrical connections, and the cable terminal section 46. Thesesections may be assembled separately and then joined together foroperation as a single unit. Likewise, the separate sections may beseparated for repair of a particular problem in a particular sectionwithout disassembly of the other sections.

The hydrophone head assembly may be disassembled by removal of screw 20and unthreading stainless steel ring 18. This removes the boot. Uponremoval of the boot theexpanded cylinder 15 and the wire cage may beremoved with removal of adapter 9. This leaves the crystal assemblyexposed which may be removed by disconnecting the wires connected tlsstflsettssdthr9 ghlunmu- The cable-gland terminal assembly may beremoved by removing screw 26 and sliding the end connector 32 away fromthe stainless steel housing 21.

It is believed that assembly of the hydrophone head assembly and thepreamplifier section are obvious from the drawing and the abovedescription. However, assembly of the cable-gland assembly is 0 believedto require explanation. The cable is of a well known type which includesa braided shield surrounded by a neoprene jacket or cover with the wiresincluded in the inner portion of the cable. Strands of the cable shield27 are unbraided and bent back over the neoprene jacket with the cableconductors 29 extending from the end of the cable longer than a s?! Thecable spacer 28 is placed over the individual cable conductors 29. Theinsulation is removed from each of the conductors even with the outersurface of the spacer 2 8 Eyelets 30 are placed over a n d soldered overthe full length of the eyelet to each wire conductor even with thesurface of the spacer 28. The ends of the conductors are fed through theapertures in the cable terminal 31 and the eyelets 'are then forced intothe apertures in the cable terminal with the bare wires extending beyondthe cable terminal. Wires 35 are soldered to the ends of the gable wiresextending beyond the cable terminal.

The cable 24 with strands of the outer s h ield 2 7 bent back over theneoprene cable jacket, the cable spacer 28, and the cable terminal 31 isslipped as an assembly into the ferrule 34 until the precision taper onthe cable terminal 31 seats in the precision taper in the end connector32. The ferrule 34 is then swaged 33 in a directed manner. which sealsand locks the cable into the end connector and electrically grounds thecable braid 27 to the cable connector. After the ferrule is swaged. theneoprene rubber 38 is simultaneoulsy formed to the end of the cableconnector.32, the ferrule 34, and the cable 24 near the ferrule, byvulcanizing in a compression mold. The wires 35 extending through theopposite end of the cable connector are soldered to the posts 41 of thehermetic-seal header. A hard setting epoxy is poured into the cableconnector then the hermetic-seal header is pressed into place and theepoxy is cured. The cable assembly is then ready for connection with thepreamplifier output connector 23.

The electrical wires 35 and those in' the hydrophone head adapterassembly have been shown as short as possible for clarity; it is ofcourse obvious that the wires must be sufficiently long to permit one tosolder them to their proper connections and then to position theelements into place.

Since the device is so small and the boot over the hydrophone head isremovable and can be placed onto the end after the cage is in position,the boot 16 may be filled with vapor free castor oil and then placedover the head assembly with any excess castor oil spilling over. Thus,no special filler plug is required, however, it is obvious that anyappropriate filler plug may be provided if desired.

The length of the hydrophone assembly is 32.38 cm, the diameter of thepreamplifier section is 2.54 cm, and each of the lithium sulfate plateshas the following dimensions: length 1.24 cm, width 0.474 cm, and thickness 0.152 cm. Therefore, the overall length of the crystal assemblywith four linearly aligned crystals is 5.08 cm including the spacing of0.04 cm between each of their adjacent ends. The overall thickness ofthe assembly is 0.304 plus the thickness of the thin electrode coatedbetween the two rows of crystals which is negligible.

FIGS. 5, 6 and 7 illustrate typical directivity patterns in the XY planewherein the center of the pattern to the top of the grid equals 50 dB.

FIGS. 8-12 illustrate typical directivity patterns in the XZ planewherein the center of the pattern to the top of the grid equals 50 dB.

FIG. 13 illustrates the typical free-field voltage sensitivity whichshows that the nominal open-circuit crystal voltage sensitivity at 0 kPagage in the frequency range Hz to 150 kHz is l95.5 dB re 1 V/p. Pa (95.5dB re 1 V/p. bar). In the frequency range 20 Hz to 150 kHz, attemperature from 3 to C and pressure to 6,895 kPa (1,000 psig) gage, thereceiving sensitivity changes by less than 1.5 dB. For any specifictemperaure from 3 to 25 C, the free-field voltage sensitivity measure inthe horizontal (XY) plane in the directions 90 or 270 C from theacoustic axis does not change with pressure to 6,895 kPa in thefrequency range 100 kHz to 500 kHz. No change in sensitivity withtemperature has been observed at pressure to 51.7 MPa (7,500 psi) gagein the frequency range 20 Hz to 4 kHz. The difference in sensitivitymeasured along the acoustic axis and at l80 to this axis in thehorizontal plane is less than 1 dB in the range 20 Hz to 150 kHz attemperature from 3 to 25 C and pressure to 6,895 kPa gage.

The above-stated characteristics are the result of present measurementcapabilities. The sensitivity of the hydrophone has been determined inclosed tanks under controlled conditions of temperature and pressure.Measurements above 4 kHz have not been made at pressure greater than6,895 kPa. However, theoretical considerations indicate that thehydrophone is capable of operating above 4 kHz at pressure to 51.7 MPawithin the frequency range and sensitivity tolerance given for 6,895kPa.

The hydrophone is omnidirectional with t 1 dB in the plane (XY) normalto its longitudinal axis up to 50 kHz; at higher frequencies, it isbidirectional. The directivity in the vertical (XY) plane is equivalentto that of a 5.08 cm line.

Operation of the hydrophone is as well known in the art. Sound wavesincident on the boot causes the castor oil to transmit vibrationalforces onto the crystals which due to their characteristics produce anelectrical potential. This is transmitted to the pre-amplifier by theconductive wires. The preamplifier increases the amplitude of theelectrical potential developed by the crystal assembly. The output ofthe preamplifier is transmitted by the conductors and cable to anydesired equipment on shore, on a ship, to a signal transmitting buoy orany other suitable equipment.

The hydrophone is suitable for broadband, highpressuretemperature-stable reference standard operation and the crystal assemblymay be easily removed for repair if needed. The tapered cable terminalpermits high pressure operation because the surrounding pressures forcethe terminal further into its seat in the end of the cable connector andthe shoulders on the eyelets prevents the conductors from expulsion.Further, the copper tube swagged to the end of the cable providesmaximum tensile strength to the cable assembly while electricallygrounding the shield to the hydrophone case.

Use of other types of materials and assembly as well as othermodifications and variations are possible in the light of the aboveteachings and should become obvious to those skilled in the art.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

l. A hydrophone which comprises:

a hydrophone head assembly,

a preamplifier electrically connected with said hydrophone head foramplifying potentials developed by said hydrophone head,

a cable,

said cable including electrical conductors connected with saidpreamplifier for conducting electrical pulses to a desired electricalinstrument,

said hydrophone head including a crystal assembly formed by two rows oflinearly aligned crystals bonded face-to-face and connected electricallyin parallel to each other and lying along the axis of said hydrophonehead assembly,

each of said linear rows including a spacing between adjacent ends ofeach of said crystals therein.

2. A hydrophone as claimed in claim 1; wherein,

each of said crystals are rectangular in shape with the crystals of onerow aligned with crystals of the other row.

3. A hydrophone as claimed in claim 2; wherein,

said crystals are supported within a wire cage in axial alignmenttherewith,

said wire cage including two U-shaped wires secured with the wires fromeach other.

4. A hydrophone as claimed in claim 3; which in- 5 cludes,

a closed end expanded metal cylinder surrounding said wire cage andconnected to electrical ground which serves as an electrostatic shieldfor the crystal assembly, N

a butyl boot surrounding said expanded meta l cylinder, l

and castor oil surrounding said crystal assembly and filling the spaceconfined by said butyl boot.

5. A hydrophone as claimed in claim 4; which includes,

a hydrophone head adapter assembly throughwhich electrical conductorspass,

means for connecting said electrical conductors with said crystalassembly, 1

an adapter secured to said head adapter assembly for easy removaltherefrom,

said wire cage and said expanded metal cylindrical cylinder connected tosaid adapter,

a stainless steel ring,

said butyl boot secured to said stainless steel ring,

said stainless steel ring adapted to be secured about said head adapterassembly for easy removal.

6. A hydrophone as claimed in claim 5; which in cludes,

. a cylindrical cable end connector,

said'cable end connector including a taperedinncr surface on one end,

a cable terminal tapered to match the taper on the innersurface of saidcable end connector,

said cable terminal including a plurality of axially aligned aperturestherethrough,

a cable, 1

said cable housing a ferrule swagged on a portion of the length from theend thereof,

, said ferrule secured to said cable connector,

electrical conductors passing through said apertures in said cableterminal,

i eyelets secured about said conductors and secured within saidapertures in said cable terminal,

I said eyelets including outwardly extending shoulders end of said cableend connector.

# l t I i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION p t760,348 Dated September 18, 1973 Robert J. Kieser et a1. Inventor(s) It:is certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Cancel the original columns 1 through 4 and substitute in their placethe corrected columns as shown on the attached sheet.

Signed and sealed this 19th clay of November 1974.

(SEAL) Attest: I

McCOY M. GIBSON JR. 4 c. MARSHALL DANN Attesting Officer Commissioner ofPatents )RM PO-1050 (10-69) USCOMM-DC wave-P09 U.S. GOVKRNMENT PRINTINGOFFKE: 9 o

3,760,,348 Dated September 18 1973 Patent No.

Robert J; Kieser et a1... Ihventofls) It is certified that error appearsin the above-identified patent and that said Letters Patent are herebyCorrected as shown below:

Cancel the original Columns 1 through 4 and substitute in their placethe corrected columns, as shown on the attached sheet.

Signed and sealed this 19th day of November. 1974.

(SEAL) Attest MCCOY M. GIBSON JR. 7 n c. MARSHALL DANN Attesting OfficerCommissioner of Patents FORM PO-IDSO (10-69) u 50 75 2 u.s. eovzmmzmPmmmc OFFICE: 93 0

1. A hydrophone which comprises: a hydrophone head assembly, a preamplifier electrically connected with said hydrophone head for amplifying potentials developed by said hydrophone head, a cable, said cable including electrical conductors connected with said preamplifier for conducting electrical pulses to a desired electrical instrument, said hydrophone head including a crystal assembly formed by two rows of linearly aligned crystals bonded face-to-face and connected electrically in parallel to each other and lying along the axis of said hydrophone head assembly, each of said linear rows including a spacing between adjacent ends of each of said crystals therein.
 2. A hydrophone as claimed in claim 1; wherein, each of said crystals are rectangular in shape with the crystals of one row aligned with crystals of the other row.
 3. A hydrophone as claimed in claim 2; wherein, said crystals are supported within a wire cage in axial alignment therewith, said wire cage including two U-shaped wires secured with the wires 90* from each other.
 4. A hydrophone as claimed in claim 3; which includes, a closed end expanded metal cylinder surrounding said wire cage and connected to electrical ground which serves as an electrostatic shield for the crystal assembly, a butyl boot surrounding said expanded metal cylinder, and castor oil surrounding said crystal assembly and filling the space confined by said butyl boot.
 5. A hydrophone as claimed in claim 4; which includes, a hydrophone head adapter assembly through which electrical conductors pass, means for connecting said electrical conductors with said crystal assembly, an adapter secured to said head adapter assembly for easy removal therefrom, said wire cage and said expanded metal cylindrical cylinder connected to said adapter, a stainless steel ring, said butyl boot secured to said stainless steel ring, said stainless steel ring adapted to be secured about said head adapter assembly for easy removal.
 6. A hydrophone as claimed in claim 5; which includes, a cylindrical cable end connector, said cable end connector including a tapered inner surface on one end, a cable terminal tapered to match the taper on the inner surface of said cable end connector, said cable terminal including a plurality of axially aligned apertures therethrough, a cable, said cable housing a ferrule swagged on a portion of the length from the end thereof, said ferrule secured to said cable connector, electrical conductors passing through said apertures in said cable terminal, eyelets secured about said conductors and secured within said apertures in said cable terminal, said eyelets including outwardly extending shoulders which prevent their removal from said apertures, and a neoprene rubber surrounding said ferrule and a portion of said cable vulcanized thereto and to the end of said cable end connector. 